Subterranean drilling involves the use of two main types of drill bits, one being a roller cone bit and the other being a fixed cutter or so-called “drag” bit. A roller cone bit has a set of cones having teeth or cutting inserts arranged on rugged bearings on the arms of the bit. As the drill string is rotated, the cones will roll on the bottom of the hole, and the teeth or cutting inserts will crush the formation beneath them. Fixed cutter or “drag” bits employ fixed superabrasive cutters (usually comprising polycrystalline diamond compacts, or “PDCs”) which crush or shear the formation as the drill string is rotated.
For both roller cone and fixed cutter bits, the economics of drilling a well are strongly reliant on the rate of penetration. Since the design of the cutting structure of a drill bit controls the bit's ability to achieve a high rate of penetration, cutting structure design plays a significant role in the overall economics of drilling a well.
Accordingly, drill bits are the subject of competitive design methodologies that seek to create a bit structure with superior performance for the particular drilling application. In general, design goals include the creation of a bit with a cutting action that is resistant to slip-stick incidents, resistant to bit whirl, and that reduces the destructive impact loads on the bit caused by down hole vibrations, thereby achieving a higher overall rate of penetration (ROP) and reduced cutter wear. To these ends, iterative design approaches are utilized to establish and test cutting structure geometries prior to manufacturing of the bit.
In one aspect, force balancing of bits is utilized to improve stabilization and bit performance. For example, each cutter exerts forces on the formation as the bit rotates and penetrates. The magnitude and direction of these forces is dependent upon cutter location, cutter engagement, back rake, and side rake. Kinematic models derived from laboratory testing are able to estimate these forces for given operating conditions and formation characteristics. Bit balance (or imbalance) can be investigated through summations of linear and moment force vectors. Adjustments to the cutter placement and orientation across the bit face may then be made to reduce the imbalance numbers in a way that results in a low summation of the lateral forces generated by each cutter. This balancing technique dramatically reduces down hole vibrations that may be caused by the bit's cutting action.
However, analysis and control of the summation of the lateral forces generated by each cutter does not consider how the individual forces generated by each cutter compare to each other. Adjacent cutters or cutters within the same region of cut may be doing substantially different levels of work and may be generating significantly different levels of forces. This can cause different rates of wear from cutter to cutter. Furthermore, where some cutters on the bit are creating significantly higher levels of force than others, significant and deleterious instantaneous force imbalances may be created as formation hardness or operating parameters change.
What is needed, therefore, is an improved design process and resulting bit cutting structure that optimizes individual cutter force, torque, work, or power distribution across the face of the bit.